On 12/29/23 15:57, David Laight wrote:
this_cpu_ptr() is rather more expensive than raw_cpu_read() since
the latter can use an 'offset from register' (%gs for x86-84).
Add a 'self' field to 'struct optimistic_spin_node' that can be
read with raw_cpu_read(), initialise on first call.
Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
---
kernel/locking/osq_lock.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 9bb3a077ba92..b60b0add0161 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -13,7 +13,7 @@
*/
struct optimistic_spin_node {
- struct optimistic_spin_node *next, *prev;
+ struct optimistic_spin_node *self, *next, *prev;
int locked; /* 1 if lock acquired */
int cpu; /* encoded CPU # + 1 value */
};
@@ -93,12 +93,16 @@ osq_wait_next(struct optimistic_spin_queue *lock,
bool osq_lock(struct optimistic_spin_queue *lock)
{
- struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
+ struct optimistic_spin_node *node = raw_cpu_read(osq_node.self);
My gcc 11 compiler produces the following x86-64 code:
92 struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
0x0000000000000029 <+25>: mov %rcx,%rdx
0x000000000000002c <+28>: add %gs:0x0(%rip),%rdx # 0x34
<osq_lock+36>
Which looks pretty optimized for me. Maybe older compiler may generate
more complex code. However, I do have some doubt as to the benefit of
this patch at the expense of making the code a bit more complex.
Cheers,
Longman
